Intentional body clipping of wide-necked basilar artery bifurcation aneurysms*

Transcription

1 J Neurosurg 93: , 2000 Intentional body clipping of wide-necked basilar artery bifurcation aneurysms* YUICHIRO TANAKA, M.D., SHIGEAKI KOBAYASHI, M.D., KAZUHIRO HONGO, M.D., TSUYOSHI TADA, M.D., HISASHI NAGASHIMA, M.D., AND YUKINARI KAKIZAWA, M.D. Department of Neurosurgery, Shinshu University School of Medicine, Matsumoto, Japan Object. Neck clipping or coil embolization cannot always achieve complete neck obstruction in wide-necked basilar artery (BA) bifurcation aneurysms. Clipping of the aneurysm body, leaving a small aneurysm rest, is one clipping method used for this kind of aneurysm to maintain the patency of the posterior cerebral arteries and perforating vessels. However, the long-term efficacy of intentional body clipping has not been well investigated. The authors reviewed their experience with intentional body clipping of wide-necked BA bifurcation aneurysms to determine suitable clipping techniques and the long-term efficacy of the procedure. Methods. Complete neck occlusion was abandoned and body clipping intentionally performed in 17 patients with BA bifurcation aneurysms; wrapping of the aneurysm rest was made in seven cases. There were 10 ruptured aneurysms (58.8%), and the size of the aneurysm was larger than 10 mm in 11 patients (64.7%). The width between the clip blades and the base of the aneurysm neck was 1 mm in 11 cases, 2 mm in four, and 3 mm in two. Favorable outcome (Glasgow Outcome Scale [GOS] Score 4 or 5) was obtained in 13 cases (76.5%) and unfavorable outcome (GOS Scores 1 3) in four cases (23.5%). Major causes of unfavorable outcome included injury to perforating arteries and major vessel occlusion following surgical manipulation, in addition to the primary damage caused by subarachnoid hemorrhage. Subarachnoid hemorrhage did not occur during a mean follow-up period of years (range years) after treatment. Conclusions. Intentional body clipping of wide-necked BA aneurysms proved to be effective to prevent subarachnoid hemorrhage, although injury to perforating arteries remains problematic. The choice of complete neck clipping or body clipping should be established early during the microsurgical procedure to reduce the risk of injury to perforating vessels. KEY WORDS aneurysm rest basilar artery cerebral aneurysm clipping N ECK clipping of BA bifurcation aneurysms is one of the most difficult microsurgical procedures, because these aneurysms are located in the deep and narrow space surrounded by the hypothalamus and brainstem. Injury to a perforating artery cannot be avoided in every case, even when the procedure is performed by experienced surgeons. 2,6,20 The BA bifurcation aneurysm has recently become one of the most popular candidates for GDC embolization because the complication rate of microsurgery in such cases is relatively high and the BA terminal is anatomically a suitable site for endovascular techniques. 3,4,13 Aneurysms having wide necks or perforating arteries that arise from the aneurysm body, however, are not appropriate for coil embolization. 3,9 To avoid occlusion or stenosis of PCAs and perforating arteries in cases of wide-necked aneurysms, we undertook body clipping, in which the aneurysm clip blades were placed over the aneurysm body a few millimeters away from the base of the neck, leaving an aneurysm rest. We retrospectively analyzed our experience to determine suitable clipping Abbreviations used in this paper: BA = basilar artery; CT = computerized tomography; GDC = Guglielmi detachable coil; GOS = Glasgow Outcome Scale; PCA = posterior cerebral artery; PCoA = posterior communicating artery; SAH = subarachnoid hemorrhage; SCA = superior cerebellar artery; VA = vertebral artery. J. Neurosurg. / Volume 93 / August, 2000 techniques and the long-term efficacy of the intentional body clipping method. Clinical Material and Methods Patient Population One hundred four patients with a BA bifurcation aneurysm were treated from 1978 to 1998 at Shinsu University Hospital and its affiliated hospitals. Neck obliteration was attempted using a microsurgical procedure in 87 patients, and coil embolization of the aneurysm was performed in 17 patients. There were 63 ruptured and 41 unruptured aneurysms. Complete neck occlusion was achieved in 70 (80.5%) of 87 microsurgical cases and body clipping was undertaken in the remaining 17 cases (19.5%). We analyzed these 17 cases for the present report. Cerebral angiography was performed before and after surgery in all cases. Cerebral angiograms including at least four different views were obtained on average 2 weeks after surgery. The maximum distance between the clip blades and the base of the aneurysm neck, which was suitable for complete neck obliteration, was measured to evaluate the size * See the Letter to the Editor and the Response in this issue in Neurosurgical Forum, p

2 Y. Tanaka, et al. FIG. 1. Case 1. Preoperative (A) and postoperative (B) VA angiograms. The postoperative angiogram (B) reveals a dog-ear residuum. The distance between the clip blades and the base of the aneurysm neck is approximately 3 mm. of aneurysm rests. A late angiographic follow-up study was not performed, except in a few cases. There were five men and 12 women with ages ranging from 39 to 71 years (mean age years). Twelve patients presented with SAH. Ten BA bifurcation aneurysms (58.8%) had ruptured. Four patients harbored aneurysms at other locations and two of these aneurysms had bled. One patient had a history of cerebral infarction and another had moyamoya disease. Aneurysms and Surgical Procedures The mean size of the aneurysms was mm (range 5 19 mm), and 11 aneurysms (64.7%) exceeded 10 mm. The aneurysm neck was located mm (range 3 15 mm) above the posterior clinoid process. Fifteen aneurysms were approached via the transsylvian route and two aneurysms via the subtemporal route. The side of the surgical approach was on the right in 10 cases and on the left in seven cases. Orbitozygomatic or zygomatic osteotomy was added to the frontotemporal craniotomy in seven cases when the aneurysm was large and/or unusual in shape or located in a high position. Temporal bridging veins were sacrificed in four cases to facilitate retraction of the temporal lobe, and the PCoA was divided in four cases. Details of the surgical techniques and selection of approach routes were reported previously. 17,27,29 Results We initially attempted complete neck occlusion in all but one patient (Case 3), in whom all four arterial branches apparently originated from the aneurysm body. Various kinds of Sugita clips and their combinations were applied to the aneurysm neck from various angles. We finally abandoned complete neck obstruction in these 16 cases, because the neck was wide and because unilateral (three cases) or bilateral PCAs arose from the aneurysm body (five cases), perforating arteries originated from the aneurysm body or close to the aneurysm neck (five cases), or perforating vessels tightly adhered to the aneurysm or were embedded in the aneurysm wall (three cases). Clip blades were intentionally placed a few millimeters distal, toward the dome side and away from the base of the aneurysm neck. The distance between the clip blades and the aneurysm neck was approximately l mm in 11 cases, 2 mm in four, and 3 mm in two cases. A thin portion of aneurysm wall was isolated from the circulation as much as possible at the time of clip placement, and the aneurysm rest was wrapped with a cotton patty (Bemsheet; Kawamoto Co., Osaka, Japan) in seven cases when the residual neck appeared fragile. Six months after treatment, each patient s outcome was evaluated using the GOS. The GOS scores were 5 in 10 cases (58.8%), 4 in three cases (17.6%), 3 in two cases FIG. 2. Case 2. Preoperative (A) and postoperative (B) VA angiograms. A long clip was applied, leaving a crescentshaped residual neck. The postoperative angiogram (B) reveals that both PCAs are occluded, possibly due to the downward dislodgment of the clip blades to the BA (C). Arrow indicates downward dislodgment. 170 J. Neurosurg. / Volume 93 / August, 2000

3 Intentional body clipping of basilar artery aneurysms FIG. 3. Case 3. Preoperative VA angiogram (A) and three-dimensional CT angiogram (B). The latter image reveals the aneurysm as viewed from behind. Both PCAs and SCAs arise from the body of the aneurysm. L = left; R = right. (11.8%), 2 in one case (5.9%), and 1 in one case (5.9%). Poor outcome scores (GOS Scores 1 3) resulted from obliteration of perforating arteries (two cases) or both major cerebral arteries and their perforating vessels (two cases), in addition to the primary damage caused by SAH. Permanent oculomotor palsy was noted in two cases. We found no SAH or mass signs of aneurysm during a mean follow-up period of years (range years) among 16 patients. One patient (Case 2) died 2 weeks after the operation as a result of a surgery-related complication caused by a downward dislodgment of the clip blades. During the follow-up period, five patients died of causes unrelated to the treated aneurysms: cardiopulmonary disease in three cases, cerebral infarction in one case, and head trauma in one case. Illustrative Cases Case 1 This 58-year-old man presented with a severe headache due to SAH. Cerebral angiography revealed a large (13- mm) wide-necked aneurysm at the BA bifurcation (Fig. 1). Neck clipping of the aneurysm was attempted via the right-sided subtemporal route. Complete aneurysm neck occlusion was impossible because several perforating arteries were found to arise at the aneurysm neck. Instead, body clipping was performed, leaving a portion of the neck unclipped to spare the perforating arteries because the remaining aneurysm wall looked thick and healthy. Postoperative angiography revealed a dog-ear residuum with the aneurysm clip blades 3 mm apart from the base of the neck. This patient returned to his previous occupation, with a mild oculomotor paresis that subsided within a couple of months. He did not experience SAH during the postoperative follow-up period, which lasted 13.5 years. Case 2 This 58-year-old man presented with dysarthria and facial paresis. A CT scan revealed a small infarction in the left putamen and a round mass in the interpeduncular cistern. Cerebral angiography demonstrated a large BA bifurcation aneurysm, which measured 16 mm (Fig. 2A). The aneurysm was approached via the right-sided subtemporal route. An initial attempt at applying a long clip kinked the PCAs and their perforating arteries. Therefore, J. Neurosurg. / Volume 93 / August, 2000 the clip blades were repositioned 2 mm apart from the neck, leaving a crescent-shaped aneurysm rest. The patient s systolic blood pressure was lowered to 80 mm Hg during clip application. Both of his pupils were noted to dilate 2 hours after the operation, with marked systemic hypertension; however, a CT scan revealed no new ischemic lesion. A CT scan obtained 1 day after the operation revealed cerebral infarctions in the thalamus and both occipital lobes. Cerebral angiography confirmed that both PCAs were occluded, possibly due to a downward dislodgment of the clip blades to the BA, occurring when the patient s blood pressure returned to the normal range (Fig. 2B). The patient did not regain consciousness and died of massive gastrointestinal bleeding 2 weeks after the operation. Case 3 This 40-year-old man presented with dizziness. A magnetic resonance image revealed an aneurysm at the distal BA. Angiography and three-dimensional CT angiography demonstrated a BA bifurcation aneurysm measuring 11 mm, with the PCAs and SCAs directly branching off from the aneurysm body (Fig. 3). The left PCA originated from the dome of the aneurysm, and the left PCoA was a fetal type. Following orbitozygomatic craniotomy, the aneurysm was exposed through the left transsylvian route. A bayonet clip with long blades was inserted between the left PCA and left SCA (Fig. 4A). The right PCA was kept on the side of the clip where the parent artery was located. A second aneurysm clip with curved blades was placed beyond the origin of the left PCA. A booster clip was finally added to the first clip. Postoperative VA angiography revealed a 3-mm-wide residuum (Fig. 4B), and left carotid artery angiography demonstrated that the distal PCA was fed through the ipsilateral PCoA (Fig. 4C). Following surgery, the patient was neurologically intact except for a temporary bout of unsteadiness. He soon returned to his previous occupation. A follow-up angiogram obtained 1 year after the operation revealed that the size of the aneurysm rest was almost stable. Discussion In their experience, Drake, et al., 6 treated a tremendous number of BA bifurcation aneurysms. They reported that 171

4 Y. Tanaka, et al. FIG. 4. Case 2. Schematic representation of the clipping method (A) and postoperative angiograms (B and C). A: A bayonet clip with long blades is inserted between the left PCA and left SCA. The right PCA is on the side of the clip where the parent artery is located. A second clip with curved blades is placed beyond the origin of the left PCA. The dotted area indicates a portion fed by the PCoA (pcom. A). B: Postoperative angiogram revealing a 3-mm-wide residual neck. C: Left carotid artery angiogram revealing no aneurysm rest and a distal PCA fed via the left PCoA. favorable outcome (excellent and good outcome) was obtained in 87.6% and unfavorable outcome (poor outcome and death) in 12.4% of patients in a series of 758 small and large aneurysms treated between 1959 and Despite the recent development of microneurosurgery, the surgical result in that series has not been surpassed by other large-numbered series. Samson and colleagues 22 reported a 24% rate of unfavorable outcome in 303 distal BA aneurysms, and Morcos and Heros 20 an 18% rate of unfavorable outcome in 124 cases. Major factors for these unfavorable outcomes included: preoperative poor grade, intraoperative aneurysm rupture, postoperative rebleeding, large aneurysm size, occlusion of the major vessel, postoperative hematoma, injury to a perforating artery, advanced age of the patient, and delayed vasospasm. 2,6,20,28 Postoperative aneurysm rupture should be prevented by complete neck occlusion; however, major vessels or perforating arteries may be occluded if complete clipping is attempted in all large and wide-necked aneurysms. Injury to a perforating artery happens more frequently in neck clipping of aneurysms at the BA bifurcation than at other locations, and has significant influence on surgical outcome. Drake, et al., reported that in 12.6% of 493 small ( 12 mm) BA bifurcation aneurysms there was injury to perforating arteries. These authors also reported that, among 758 nongiant aneurysms, the rate of unfavorable outcome (30.8%) in patients with injury to perforating arteries was three times greater than that (9.5%) in patients without such an injury. Surgical outcome in the present series was not better than those of previous reports, despite the use of intentional body clipping for securing vessels. 6,20,22 One of the reasons for this is that the majority of BA aneurysms in the present series were large and/or wide necked. Aneurysm size is one of the most important predictive factors of surgical outcome in the BA bifurcation aneurysms, and vascular occlusion has been known to be the most common source of permanent morbidity in patients with large aneurysms. 6,20,28 The attempt to pursue complete clipping may have led us to injure perforating arteries before we decided to abandon complete neck occlusion. We should have recognized that complete clipping was impossible during the early stage of the microsurgical procedure. Recently, we started to use aneurysmography, digital angiotomosynthesis, and three-dimensional CT angiography to determine the fine anatomy of the aneurysm and its surrounding vessels for possible body clipping. 12,23 Endovascular treatment of selected cerebral aneurysms by using GDCs has become accepted as an alternative procedure to clipping surgery. 3,4,13 The long-term outcome of GDC treatment is not well defined, but some patients have undergone aneurysm recanalization or coil compaction during early and late follow-up periods, with the potential of rebleeding. 3,4,8,19 Bavinzski, et al., 3 reported coil compaction in 12 (38.7%) of 31 cases during follow-up periods ranging between 2 to 72 months, and recanalization was observed in 57% of the large aneurysms. Byrne, et al., 4 reported that recurrent filling was noted in 38 (14.7%) of 259 aneurysms during a median follow-up period of 22.3 month, and that rebleeding occurred in three (7.9%) of the 38 recurrent aneurysms. Eskridge and Song 8 found 3.3% and 4.1% rebleeding rates in 83 ruptured and 67 unruptured BA bifurcation aneurysms, respectively, during a follow-up period lasting approximately 1 year after GDC embolization. Fernandez Zubillaga and colleagues 9 analyzed the correlation between the neck size and degree of aneurysm occlusion by GDCs in 20 small- ( 4 mm) and 51 wide- ( 4 mm) necked aneurysms. More than 90% of aneurysm necks were wide in the large-and-giant-aneurysm population, and complete aneurysm occlusion was observed on angiography in only 15.7% of wide-necked aneurysms. Complete neck obliteration seems difficult to achieve using standard GDC placement in the majority of large or wide-necked aneurysms, although Moret, et al., 21 reported the effectiveness of a remodeling technique in which a nondetachable balloon is temporarily inflated in front of the neck during each coil placement in wide-necked aneurysms. Sindou and coworkers 24 found aneurysm remnants in 18 (5.9%) of 305 aneurysms at various sites after clipping surgery. A wide neck was estimated to be the most common cause of incomplete clipping. Samson, et al., 22 reported that residual aneurysm was revealed by postoperative angiography in 6% of microsurgically treated BA apex aneurysms. These authors did not experience rebleeding of the aneurysms during an average follow-up period of J. Neurosurg. / Volume 93 / August, 2000

5 Intentional body clipping of basilar artery aneurysms J. Neurosurg. / Volume 93 / August, 2000 years, although four of 11 such aneurysms underwent reoperation because the residual rest grew during that period. Feuerberg, et al., 10 examined late angiographic findings of aneurysm sacs that had been incompletely obliterated by clipping surgery. Angiographic follow-up studies of 21 such aneurysms for 6 years revealed that one aneurysm rest increased in size and bled, five were spontaneously obliterated, two decreased in size, and 13 remained unchanged. An annual rate of rebleeding from an aneurysm rest was calculated to be 0.6%. David and colleagues 5 reported a late angiographic study of surgically treated aneurysms; there were eight dog-ear residua, two of which were recognized at surgery and reinforced with cotton patties. Of the eight dog-ear residua, six (75%), including the two reinforced with cotton patties, remained stable and the other two (25%) enlarged slightly, one of which came to the authors attention after the patient suffered an SAH. An annual hemorrhage risk was calculated to be approximately 1.9%. Among approximately 1900 patients, Lin, et al., 18 reported 19 cases of aneurysms that grew from an aneurysm rest, and 15 of these patients presented with recurrent SAH. Six of the 19 aneurysms were located at the BA bifurcation. These grew to be 3- to 25-mm aneurysms from 1- to 2-mm residual necks between 3 and 13 years (mean 6.3 years); four of these aneurysms rebled. The patients who harbored these six BA bifurcation aneurysms were young, with ages ranging from 30 to 48 years (mean 37 years) at the time of the first surgery. All the aneurysms had been small at the initial surgery, and none of them had been reinforced with wrapping materials. According to the report by investigators of the International Study of Unruptured Intracranial Aneurysms, 16 the rupture rate for patients with unruptured aneurysms 10 mm or larger in diameter was approximately 20 times the rate for smaller aneurysms. Body clipping can prevent early postoperative rupture in patients who have experienced ruptured aneurysms because the rupture site is excluded from the circulation. It also is not illogical to estimate that, after clipping the body of a large aneurysm, the aneurysm rest would have at most the same risk of rupture as an unruptured small aneurysm. Body clipping is not a perfect method to prevent rebleeding, but the amount of prevention it provides large or wide-necked aneurysms seems better than that provided by GDC treatment. Body clipping has an advantage in that every portion of an aneurysm is inspected directly under a microscope and a weak portion of aneurysm wall can be eliminated as much as possible by using a clip. The efficacy of aneurysm wrapping, which is used to supplement a clip, remains unclear; however, we believe such reinforcement of the aneurysm rest is effective in reducing the risk of rebleeding. 30 Greater closing force is often required of the aneurysm clip for body clipping of large aneurysms than for regular neck clipping, because the diameter of the body portion is apparently longer than that of the aneurysm neck. Dujovny, et al., 7 reported experimental results showing that three times more closing force is necessary when the vessel diameter is doubled. Even if one chooses a long clip, a strong closing force cannot be expected because the closing force measured 2 mm from the tip of blades is almost constant, regardless of blade length. 1,26 Note that the closing force of Sugita clips is slightly weaker than that of some other clips. The opening and closing pressures of Sugita clips are set lower than other clips to increase maneuverability and to decrease tissue damage in the possible event that the clip blades must be replaced. If the patient s blood pressure is maintained at a low level during clip application, as occurred in Case 2, blood pressure should be returned to the normal range soon after clip application. Experimental findings indicate that the minimal occlusion force required of clips at a blood pressure of 190 mm Hg is twice that required at a blood pressure of 90 mm Hg. 7 We should have reinforced the closing pressure of the clip applied in Case 2 by placing another clip in a parallel fashion or by adding a booster clip. Although angiographic follow-up review was not performed in all patients for various reasons, at least subsequent bleeding did not occur during 118 patient years in the present study. In our experience and that of others, large-to-giant aneurysms located at the BA terminal are likely to rupture eventually, and this form of treatment may be justified for large and wide-necked BA bifurcation aneurysms. 15,25 In addition to intentional body clipping, open surgery also allows direct inspection of the aneurysm, by which one can treat it either by occluding only the bleb or by partial clipping for subsequent coil embolization if necessary. 11,14 References 1. Atkinson JLD, Anderson RE, Piepgras DG: A comparative study in opening and closing pressures of cerebral aneurysm clips. Neurosurgery 26:80 85, Batjer HH, Samson DS: Causes of morbidity and mortality from surgery of aneurysms of the distal basilar artery. Neurosurgery 25: , Bavinzski G, Killer M, Gruber A, et al: Treatment of basilar artery bifurcation aneurysms by using Guglielmi detachable coils: a 6-year experience. J Neurosurg 90: , Byrne JV, Sohn MJ, Molyneux AJ, et al: Five-year experience in using coil embolization for ruptured intracranial aneurysms: outcomes and incidence of late rebleeding. J Neurosurg 90: , David CA, Vishteh AG, Spetzler RF, et al: Late angiographic follow-up review of surgically treated aneurysms. J Neurosurg 91: , Drake CG, Peerless SJ, Hernesniemi JA: Surgery of Vertebrobasilar Aneurysms: London, Ontario Experience on 1767 Patients. Vienna: Springer-Verlag, 1996, pp Dujovny M, Kossovsky N, Kossowsky R, et al: Intracranial clips: an examination of the devices used for aneurysm surgery. Neurosurgery 14: , Eskridge JM, Song JK, and the Participants: Endovascular embolization of 150 basilar tip aneurysms with Guglielmi detachable coils: results of the Food and Drug Administration multicenter clinical trial. 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6 Y. Tanaka, et al. rysmographic studies: a perforating artery originating from the dome of a basilar tip aneurysm. AJNR 18: , Guglielmi G, Viñuela F, Duckwiler G, et al: Endovascular treatment of posterior circulation aneurysms by electrothrombosis using electrically detachable coils. J Neurosurg 77: , Hacein-Bey L, Connolly ES Jr, Mayer SA, et al: Complex intracranial aneurysms: combined operative and endovascular approaches. Neurosurgery 43: , Inagawa T, Hada H, Katoh Y: Unruptured intracranial aneurysms in elderly patients. Surg Neurol 38: , The International Study of Unruptured Intracranial Aneurysms Investigators: Unruptured intracranial aneurysms risk of rupture and risks of surgical intervention. N Engl J Med 339: , Kobayashi S, Tanaka Y: Aneurysm clip design, selection, and application, in Apuzzo MLJ (ed): Brain Surgery: Complication Avoidance and Management. New York: Churchill Livingstone, 1993, pp Lin T, Fox AJ, Drake CG: Regrowth of aneurysm sacs from residual neck following aneurysm clipping. J Neurosurg 70: , Mericle RA, Wakhloo AK, Lopes DK, et al: Delayed aneurysm regrowth and recanalization after Guglielmi detachable coil treatment. Case report. J Neurosurg 89: , Morcos JJ, Heros RC: Distal basilar artery aneurysm: surgical techniques, in Batjer HH (ed): Cerebrovascular Disease. Philadelphia: Lippincott-Raven, 1997, pp Moret J, Cognard C, Weill A, et al: [Reconstruction technique in the treatment of wide-neck intracranial aneurysms. Longterm angiographic and clinical results. Apropos of 56 cases.] J Neuroradiol 24:30 44, 1997 (Fr) 22. Samson D, Batjer HH, Kopitnik TA Jr: Current results of the surgical management of aneurysms of the basilar apex. Neurosurgery 44: , Shigeta H, Sone S, Kasuga T, et al: Digital angiotomosynthesis for preoperative evaluation of cerebral arteriovenous malformations and giant aneurysms. AJNR 15: , Sindou M, Acevedo JC, Turjman F: Aneurysmal remnants after microsurgical clipping: classification and results from a prospective angiographic study (in a consecutive series of 305 operated intracranial aneurysms). Acta Neurochir 140: , Sugita K: Microneurosurgical Atlas. Berlin: Springer-Verlag, 1985, pp Sugita K, Hirota T, Iguchi I, et al: Comparative study of the pressure of various aneurysm clips. J Neurosurg 44: , Sugita K, Kobayashi S, Shintani A, et al: Microneurosurgery for aneurysms of the basilar artery. J Neurosurg 51: , Tanaka Y, Kobayashi S, Kyoshima K, et al: Factors influencing surgical outcome of the basilar bifurcation aneurysms. Neurol Med Chir Suppl 38:79 82, Tanaka Y, Kobayashi S, Sugita K, et al: Characteristics of pterional routes to basilar bifurcation aneurysm. Neurosurgery 36: , Todd NV, Tocher JL, Jones PA et al: Outcome following aneurysm wrapping: a 10-year follow-up review of clipped and wrapped aneurysms. J Neurosurg 70: , 1989 Manuscript received October 22, Accepted in final form April 28, Address reprint requests to: Kazuhiro Hongo, M.D., Department of Neurosurgery, Shinshu University School of Medicine, Asahi, Matsumoto , Japan. 174 J. Neurosurg. / Volume 93 / August, 2000